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The stress() function will start spawning threads which will >> operate on the destination location, triggering uffd-operations like >> UFFDIO_COPY from src to dst, which means that we will require 64 more >> hugepages for the dst location. >> >> Let us observe the locking_thread() function. It will lock the mutex kept >> at dst, triggering uffd-copy. Suppose that 127 (64 for src and 63 for dst) >> hugepages have been reserved. In case of BOUNCE_RANDOM, it may happen that >> two threads trying to lock the mutex at dst, try to do so at the same >> hugepage number. If one thread succeeds in reserving the last hugepage, >> then the other thread may fail in alloc_hugetlb_folio(), returning -ENOMEM. >> I can confirm that this is indeed the case by this hacky patch: >> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >> index 753f99b4c718..39eb21d8a91b 100644 >> --- a/mm/hugetlb.c >> +++ b/mm/hugetlb.c >> @@ -6929,6 +6929,11 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte, >> >> folio = alloc_hugetlb_folio(dst_vma, dst_addr, false); >> if (IS_ERR(folio)) { >> + pte_t *actual_pte = hugetlb_walk(dst_vma, dst_addr, PMD_SIZE); >> + if (actual_pte) { >> + ret = -EEXIST; >> + goto out; >> + } >> ret = -ENOMEM; >> goto out; >> } >> >> This code path gets triggered indicating that the PMD at which one thread >> is trying to map a hugepage, gets filled by a racing thread. >> >> Therefore, instead of using freepgs to compute the amount of memory, >> use freepgs - 10, so that the test still has some extra hugepages to use. >> Note that, in case this value underflows, there is a check for the number >> of free hugepages in the test itself, which will fail, so we are safe. >> >> Signed-off-by: Dev Jain >> --- >> tools/testing/selftests/mm/run_vmtests.sh | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh >> index 471e539d82b8..6a9f435be7a1 100755 >> --- a/tools/testing/selftests/mm/run_vmtests.sh >> +++ b/tools/testing/selftests/mm/run_vmtests.sh >> @@ -326,7 +326,7 @@ CATEGORY="userfaultfd" run_test ${uffd_stress_bin} anon 20 16 >> # the size of the free pages we have, which is used for *each*. >> # uffd-stress expects a region expressed in MiB, so we adjust >> # half_ufd_size_MB accordingly. >> -half_ufd_size_MB=$(((freepgs * hpgsize_KB) / 1024 / 2)) >> +half_ufd_size_MB=$((((freepgs - 10) * hpgsize_KB) / 1024 / 2)) > Why 10? I don't know much about uffd-stress but the comment at the top says it > runs 3 threads per CPU, so does the number of potential races increase with the > number of CPUs? Perhaps this number needs to be a function of nrcpu? Yes the race will amplify with nrcpus, technically we need nr_cpus - 1 extra hugepages - the worst case is that all threads will try to perform uffd-copy on the same address, one of them will reserve the last hugepage and others will fail. 10 was just a random number; I see that run_vmtests.sh already has nr_cpus computed so I can easily reuse that. I'll send a v2. > > I tested it and it works though so: > > Tested-by: Ryan Roberts Thanks. > >> CATEGORY="userfaultfd" run_test ${uffd_stress_bin} hugetlb "$half_ufd_size_MB" 32 >> CATEGORY="userfaultfd" run_test ${uffd_stress_bin} hugetlb-private "$half_ufd_size_MB" 32 >> CATEGORY="userfaultfd" run_test ${uffd_stress_bin} shmem 20 16